The need for novel abrasion resistant coatings that are transparent and can withstand out-of-door weathering is for instance obviated by the ever increasing reliance on plastics as convenient substitutes for glass glazing in many out-of-door applications. Plastics, such as polyesters, polycarbonates and polymethacrylates are rapidly replacing glass in applications ranging from motorized vehicles and multistory buildings, to optical lenses for eyeglasses and precision instruments.
Although plastics have decided advantages over glass by virture of their lighter weight and increased shatter resistance, they suffer from two main drawbacks. They are much softer than glass and they are easily damaged by everyday exposure to abrasives, such as dust and cleaning equipment. They also can be severely damaged by environmental conditions such as exposure to solvents, to sunlight and to fluctuations in temperature and humidity.
It is no surprise therefore that considerable efforts have been made in the past to improve the durability of plastic articles to the dual effects of weathering and exposure to abrasives. The most commonly used method for improving the abrasion resistance of plastics is application of a top coat of a resin specifically formulated to be abrasion resistant. In most, if not all cases, the most desirable top coats for plastic articles are photochemically cured resins of high crosslink density. In particular, thermally cured protective coatings are not desirable for plastic substrates on account of the fact that the majority of thermosetting coatings require a heat cure cycle at temperatures which can cause thermal distortion or even degradation of the substrate.
Soft plastics are often protected from marring or scratching by application of a hardenable top coat especially formulated to be abrasion resistant. Examples of the most successful top coats for this purpose are certain room-temperature curing silicone resins derived from functionalized silane monomers, such as disclosed in U.S. Pat. No. 4,049,861.
U.S. Pat. Nos. 3,976,497; 3,986,997; 3,708,285; 4,368,236; 4,368,235; 4,478,876; and 4,491,508 disclose that the scratch resistance of polymers can be enhanced by coatings derived from hydrolyzable silanes and polymers derived from a combination of acryloxy functional silanes and polyfunctional acrylate monomers. The scratch resistance of these polymers can be further enhanced by addition of colloidal silica. Most likely this improvement is due to a combination of two effects brought on by the addition of colloidal silicon dioxide particles: (1) the mere substitution of part of the soft silicone resin by the much harder inorganic oxide particles that are trapped in the polymer matrix and (2) changes in the mechanical properties of the coating as a result of the "crosslinking" effect of the inorganic oxide. Whatever the mechanism may be, the addition of colloidal silica to hydrolyzable silanes and functionalized silanes is a straightforward process because the polycondensation of the siloxane is carried out in the aqueous environment of the colloidal silica. There are, however, several drawbacks associated with the utilization of hydrolyzed silanes or functionalized silanes in the formulation of abrasion resistant coatings. The first is that the shelf-life of the partially hydrolyzed silanes is often limited due to the progressive gelling of the condensation polymer, as is documented in U.S. Pat. No. 3,986,997 for example. Another drawback is that full cure of the silesquioxane to yield the highly crosslinked silicone matrix is often an extremely slow process that is only partially remedied by a catalyst or by heating. As a result of the slow cure kinetics, silane based abrasion resistant coatings are often susceptible to shrinkage and ultimately to stresscracking. Lastly, silane based hard coats suffer from poor adhesion to plastic substrates, such as polymethyl methacrylate, and require therefore some form of physical or chemical priming of the adherent surface.
Attractive alternatives to silane-based hard coats for protecting plastic substrates are compositions containing in polymerized form, one or more acrylate or methacrylate functionalities on a monomer, oligomer or resin. The popularity of acrylic or methacrylic based hard coats is due to at least three reasons: 1) they yield transparent films, and are therefore ideal for protecting substrates that need remain transparent, or possess a color that need to be visible, 2) for decorative or other functional reasons, they are easily cured at room temperature by exposure to U.V. or electron beam irradiation, which qualifies them as top coats for heat sensitive substrates, and 3) they are capable of yielding films with high crosslink density (and therefore high hardness) by virtue of the fact that more than one crosslinkable functionality may be attached to a given monomer or a given oligomer chain.
Abrasion resistant coating compositions based on multifunctional acrylate or methacrylate monomers have been disclosed in the prior art. In U.S. Pat. No. 3,968,305 there is described a plastic shaped article having a scratch-resistant polymer surface layer consisting of, in polymerized form, (a) 20 to 100 weight percent of a compound having a total of at least 3 acryloxy and/or methacryloxy groups linked with a straight chain aliphatic hydrocarbon residue and (b) 0 to 100 weight percent of at least one copolymerizable mono- or diethylenically unsaturated compound and (c) a crosslinking catalyst for thermal or U.V. radiation cure. In U.S. Pat. No. 3,968,309 there is disclosed a mar-resistant coating composition comprising at least 30 percent by weight of at least one polyfunctional methacryloyloxy or acryloyloxy compound to which is added 0.01 to 5% by weight of a fluorine-containing surfactant.
In U.S. Pat. Nos. 4,198,465 and 4,262,072 there are disclosed abrasion resistant coating formulations containing polyfunctional acrylate and/or methacrylate monomers having heterocylic hydantoin groups in the backbone and capable of hardening by exposure to U.V. light.
Despite their advantages in so far as ease of crosslinking, transparency, and hardness after cure, the abrasion resistance of top coats derived from polyfunctional acrylates or methacrylates still leaves much room for improvement, and is far inferior to the abrasion resistance of glass or most ceramic materials.
In U.S. Pat. No. 4,499,217 there are disclosed thermoset resin liquid compositions containing colloidal silica that had been freed from water and redispersed in alcohol prior to mixing with the resins. An example of an acrylic resin is used. Although the dry cured film of this composition was reported to exhibit enhanced abrasion resistance, the curing conditions required for thermosetting resins in general, including those reported in U.S. Pat. No. 4,499,217 preclude them from being used as scratch-resistant coatings for the great majority of common plastic substrates. Depending on the nature of the thermosetting resin curing times ranging from half an hour to several hours at temperatures often well exceeding 100.degree. C. are required. Such conditions are conducive to softening, distortion and/or degradation of commercially important plastics such as polymethyl methacrylate, polyesters, polyolefins, and polycarbonates. The compositions disclosed in this patent use polymerizable resins in order to achieve high molecular weights of the coating. No monomers are used.
Photocurable abrasion resistant coating compositions comprising a non-aqueous dispersion of colloidal silica in polyethylenically-unsaturated monomer or its use to provide substrates with an improved abrasion resistant coating have not been disclosed.